Advanced photonic integrated circuits require large-scale integration of high-speed electro-optic (EO) functional components on a chip. Low power consumption and high operation speed are thus key metrics for almost all integrated EO devices. Here, we demonstrated a ring resonator modulator based on lead zirconate titanate (PZT) on a SiO2/Si substrate. The ridge waveguides were employed to keep a large spatial overlap between the optical field and the electric field within the PZT layer. The device exhibits a data rate of 56 Gbit/s and significant tuning efficiency, reaching up to 35.8 pm/V, corresponding to 1.17 V·cm. The demonstration of energy efficient and high-speed EO modulation paves the way for realizing dense PZT photonics integrated circuits.

Global data traffic has witnessed an exponential increase with the fast development of 5G/B5G, artificial intelligence, cloud computing, and Internet of things.1,2 Optical interconnection is a promising option to overcome transmission bottlenecks in data centers and high-performance computers. In the optical interconnection, an electro-optic (EO) modulator is a vital component that plays an information encoding engine from electrical to optical domain to ensure a high-speed data transmission.3 Two types of EO modulators are most intensively studied: Mach–Zehnder interferometer (MZI) modulators and ring resonator modulators.4–7 Compared to MZI, the advantages of ring modulators are their compact footprint, low driving voltage, and the ability to drive them as lumped RF elements that removes the need for traveling-wave design. Moreover, their wavelength selectivity enables dense wavelength division multiplexed links at the micrometer scale and consequently increases aggregate link bandwidth drastically.8–10 

Up until now, ring modulators have been realized by using a number of material platforms, such as silicon (Si), EO polymer, and crystals.11–13 Recent progress of Si modulators relying on the free-carrier plasma-effect is of interest and offers a realistic means of achieving ultra-fast data transfer, enabled by a careful balance among the doping configuration, loss control, and free-carriers’ lifetime.14 EO polymer benefiting from the strong Pockels effect with a high EO coefficient r33 > 100 pm/V can achieve outstanding linear modulation and low power-consumption.15 However, the thermal and chemical stability of organic materials still needs to be validated. “EO ferroelectric thin-film on insulator” has emerged recently as a new candidate for the next generation of high-speed optical interconnect devices.7,16–18 In this platform, a sub-micrometer-thick ferroelectric thin-film also with the Pockels effect is preferably bonded on top of a low-index SiO2 substrate, and the waveguides are created by dry etching of the ferroelectric thin-film. This platform allows for an integration of photonic devices and high-speed electronic circuits, simultaneously enabling strong modal confinement and effective overlap between optics and electric signals.19 Lithium niobate (LN) or barium titanate (BTO) thin-film ring modulators have been proven as an effective path to realize strong EO modulation within integrated photonics. For example, a high speed of 40 Gbit/s and a low VπL of 2.2 V·cm have been achieved in the LN ring modulator13 and 40 Gbit/s and 0.45 V·cm in BTO racetrack resonator modulators.20 Yet, these ring structures remain non-ideal due to relatively low EO tunability or high propagation loss. Caused by the relatively low EO coefficient as 30 pm/V, the EO tunability (defined as ∆λi/∆V, where ∆λi is the resonance peak shift and V is the applied DC voltage) of the LN ring modulators is <10 pm/V.21,22 Although BTO has a high EO coefficient, the high waveguide propagation loss ∼10 dB/cm is still a major issue.20,23 In addition, there are other concerns regarding the requirement of complex bonding fabrication of LN and BTO thin-films.

Lead zirconate titanate PbZrxTi1−xO3 (PZT) thin-film is another promising candidate for EO modulators because of many desirable characteristics, such as a high EO coefficient, excellent optical transparency, and a straightforward chemical solution deposition (CSD) fabrication process.24 PZT thin-film on silicon nitride (SiN) ring modulator has been realized with a data rate up to 40 Gbit/s and a low propagation loss of 1 dB/cm.17 Despite this progress, some properties need to be improved further for the widespread utilization of the PZT modulators. These include a VπL of 3.2 V·cm, a relatively low in-device EO coefficient of 60 pm/V, and an estimated extinction ratio of merely ∼1 dB at 40 Gbit/s modulation. Recently, we have explored “PZT ferroelectric thin-film on insulator (PZTOI)” and realized low propagation loss waveguides via the traditional photolithography and inductively coupled plasma (ICP) etching.25 The obtained EO coefficient of the PZTOI is ∼100 pm/V at the wavelength of 1550 nm, which leads to the decrease of VπL and the improvement of high-speed modulating quality. Furthermore, the refractive index contrast between PZT and SiO2 is around 1.0. Such a large contrast may contribute to a small bending radius, which allows for a compact ring resonator and denser photonic circuit layout.

In this work, we have achieved high tuning efficiency and high-speed ring modulators on the PZTOI platform. The PZT thin film was fabricated by the chemical solution method on a SiO2/Si substrate. The thin films can be directly deposited on substrates with high quality and low cost. The designed structure not only confines optical mode tightly in the PZT waveguide core but also effectively shorten the inter-electrode gap. The fabricated device is amenable to achieve a high EO effect, demonstrating a record high EO coefficient of 100 pm/V. The ring modulator shows an EO tunability of ∆λi/∆V = 35.8 pm/V, corresponding to VπL = 1.17 V·cm. A high-speed test of the modulator reveals an on–off keying (OOK) modulation up to 56 Gbit/s with an extinction ratio of ∼3.1 dB.

The structures of the ring modulator are shown in Figs. 1(a) and 1(b), where the waveguide consists of SiO2 claddings, a PZT layer, aluminum (Al) co-planar electrodes, and a Si substrate. The refractive indices of SiO2 and PZT are 1.45 and 2.40 at the wavelength of 1.55 µm, respectively. The thicknesses of top and bottom SiO2 claddings are around 1.0 and 2.0 µm, respectively. The PZT ridge is totally 350 nm thick with a rib width of 1.0 µm for the single mode operation. For optimizing the device performance, the etching depth (de) and inter-electrode gap (ginter) are carefully determined. Figure 1(b) shows the simulated optical TE mode-field and DC electric-field distribution in the device, indicating the large overlap between the optical and DC fields. Compared to previous ring modulator17 on the PZT platform, a thicker PZT thin film can be fabricated on the SiO2/Si substrate, and the ridge waveguide can distribute more light within the PZT layer. 80.9% of the optical field is distributed in the PZT layer, while 7.1% is found in the substrate and 12% is found in the top cladding layer. Therefore, a higher tuning efficiency and a high extinction ratio are achievable.

FIG. 1.

(a) Top view of a ridge PZT waveguide ring modulator and cross section of the ridge PZT waveguide. (b) Schematic of the ridge PZT waveguide. The fundamental TE optical mode is plotted in false colors. The quiver plot shows the applied electric field distribution between the Al electrodes. (c) Simulation EO overlap factor of the ring modulator. (d) Simulation of the tuning efficiency of the ring modulator. (e) Simulation of the propagation loss. The red rectangles show the approximate parameters used in this work.

FIG. 1.

(a) Top view of a ridge PZT waveguide ring modulator and cross section of the ridge PZT waveguide. (b) Schematic of the ridge PZT waveguide. The fundamental TE optical mode is plotted in false colors. The quiver plot shows the applied electric field distribution between the Al electrodes. (c) Simulation EO overlap factor of the ring modulator. (d) Simulation of the tuning efficiency of the ring modulator. (e) Simulation of the propagation loss. The red rectangles show the approximate parameters used in this work.

Close modal
The EO tunability ∆λi/∆V = Δnpzt · LV (L is the length of the electrode, 281 µm) is attributed to the PZT refractive index change (∆npzt), which corresponds to the in-device EO response of the ring resonator.5,17 Theoretically expected ∆npzt can be expressed as
Δnpzt=12npzt3rΔVginterΓ,
(1)
Γ=ginterVε0cnpztpztExeExop2dxdyRe(EOP×HOP*) ⋅eẑdxdy,
(2)
where npzt is the PZT refractive index, r is the PZT in-device EO coefficient (estimated 130 pm/V24), V is the voltage, Γ is the EO overlap factor, ɛ0is the vacuum permittivity, and c is the speed of light in vacuum. Exe is the in-plane (x direction) component of the electric field, and Exop represents the in-plane (x direction) transversal component of the optical field. According to Eq. (1), narrow-gap electrodes are beneficial to achieve a high ∆λi/∆V. In addition, as shown in Fig. 1(c), a shallow etching depth (smaller de) is preferred to concentrate the optical mode in the active PZT core as much as possible (higher overlap factor). Meanwhile, a smaller de will inevitably cause a high absorption loss αabs from the narrow-gap co-planar electrodes due to limited confinement of the optical mode. The Q factor of the ring resonator is determined by the waveguide loss. The device 3-dB modulation bandwidth (f3dB) is determined by the RC time and the photon lifetime as expressed by 1/(f3dB)2 = (2πτ)2 + (2πRC)2, where τ = λQ/(2πc) is the cavity photon lifetime (c is the light speed in vacuum), R is the contact resistance of the electrode, and C is the device capacitance. Since the PZT thin film acts as an insulator, the RC time is minimal, and the high-frequency response is primarily limited by the cavity photon lifetime needed to accumulate and release energy within the ring resonator. To achieve a wider bandwidth, it is advantageous to have a low Q factor ring modulator. However, excessively low Q factor may lead to poor extinction ratios, compromising the quality of high-speed modulation signals. Therefore, careful control of the loss is essential to attain a suitable Q factor. In Figs. 1(d) and 1(e), the calculated tuning efficiency ∆λi/∆V and loss αabs are plotted as a function of de and of ginter, respectively. There is a clear trade-off between ∆λi/∆V and αabs within the de range of 100–350 nm and ginter of 2.0–5.0 µm. By taking into account such a trade-off, we chose etching depth de = 150 nm and electrode gap ginter = 3.5 µm to minimize αabs to 2.2 dB/cm (including electrode absorption and waveguide loss) and achieve a high ∆λi/∆V as 46.8 pm/V.

According to the design above, the PZT ring modulator was fabricated as follows: First, the PZT film was obtained via the chemical solution deposition on a 2-µm silicon dioxide layer sitting on a silicon (from JuheEO).26 Subsequently, the micro-ring pattern was defined with the metal (Ti+Ni) mask, which was obtained by the electron-beam lithography and lift-off method. The SF6 plus Ar+ plasma milling method was used to etch the PZT layer for forming the ring resonator. After the metal mask was removed by RCA (NH4OH, H2O2, H2O mixed by the volume ratio of 1:1:5), the co-planar Al electrodes were deposited via the electron-beam evaporator and lift-off. Finally, the sol-gel SiO2 top cladding was spin-coated and then baked on a hotplate at 150 °C for 120 min. The PZT thin film has been annealed in the tube at a temperature of 600 °C. Therefore, the handing temperature of 150 °C has no impact on the device. Figure 1(a) illustrates the scanning electron microscope (SEM, Zeiss Gemini500) images of the fabricated modulator. To induce the strong EO effect in PZT thin-film, the ring modulator was poled by applying a DC field of 15 V/μm across the device at room temperature.17 

The transmission spectra of the ring resonator were measured by using an end-face coupling system. Light from a tunable laser (TSL-510, Santec) was coupled into the waveguide through a polarization-maintaining fiber. A polarizer located between the laser and the fiber was used to control the input light as TE polarization. The output light from the waveguide was collected by another fiber and then detected via a photodetector (MPM240H, Santec). DC bias voltages from −2 to 10 V generated from a digital source meter (2410, Keithley) were loaded onto the modulator for the ∆λi/∆V test.

Figure 2(a) shows the measured transmission spectrum of the fabricated ring resonator by scanning the wavelength of the tunable laser. From the measured result, the resonator has a Q factor of ∼8000 and an extinction depth of ∼13 dB. The ∆λi/∆V was determined by measuring the spectral shift at one resonant peak as shown in Fig. 2(b). The high-resolution spectra were fitted by the Lorentz function based on the measured points. As can be observed in the spectral change, the extinction ratio varies less than 0.2 dB, and the Q factor almost maintains constant. The resonance wavelength shift is plotted as a function of voltage, and the linear regression as shown in Fig. 2(c) provides a ∆λi/∆V of 0.0358 nm/V. From this, the half-wave voltage–length product was estimated to be VπL=LλFSRV/(2Δλi)=1.17 V·cm, where L is 281 µm and λFSR is 3 nm. Based on ∆λi/∆V, we calculated the in-device EO coefficient of 100 pm/V in the PZT ring modulator.17 

FIG. 2.

(a) Normalized transmission spectrum of a PZT ring modulator. (b) Transmission spectra for various DC voltages. (c) Resonance wavelength shift as a function of voltage.

FIG. 2.

(a) Normalized transmission spectrum of a PZT ring modulator. (b) Transmission spectra for various DC voltages. (c) Resonance wavelength shift as a function of voltage.

Close modal

The high-speed modulation of the modulator was characterized by using the setup as shown in Fig. 3(a). The input laser was fixed at one resonant wavelength. An arbitrary waveform generator (AWG) (N4975A, Keysight) with non-return-to-zero electrical signals and a driver amplifier (83051A, Keysight) were used to drive the device. The peak-to-peak driving voltage Vp–p was 3.0 V. The modulated light, after passing an erbium-doped fiber amplifier (EDFA) and a bandpass filter (BPF), was measured by using an optical sampling oscilloscope with a 45-GHz optical bandwidth (N1092, Keysight). Figure 3(b) exhibits the measured optical eye diagrams of 32 and 56 Gbit/s with clear eye openings. Extinction ratios of these eye diagrams were measured to be ∼5.8 and ∼3.1 dB. The calculated bandwidth is 24.9 GHz, and the bandwidth can be improved by taking the detuning wavelength method.27 The modulation speed was limited by the AWG (25 GHz bandwidth), rather than by the PZT ring modulator itself.

FIG. 3.

(a) Sketch of the setup used for the eye diagram measurements. (b) Eye diagrams of a PZT ring modulator, measured with a non-return-to-zero scheme (211 − 1 pseudorandom binary sequence) and a peak-to-peak drive voltage of 3.0 V.

FIG. 3.

(a) Sketch of the setup used for the eye diagram measurements. (b) Eye diagrams of a PZT ring modulator, measured with a non-return-to-zero scheme (211 − 1 pseudorandom binary sequence) and a peak-to-peak drive voltage of 3.0 V.

Close modal

As presented above, the ring modulator on the PZTOI platform demonstrated here can achieve high tuning efficiency and high speed. The PZT thin film can be fabricated on a SiO2/Si substrate by the chemical solution method. The manufacturing procedure is highly scalable and low cost. Meanwhile, the PZT thin film has a large EO coefficient and a wide transparent window. In the past years, researchers found that ferroelectric thin film such as lithium niobate is a kind of excellent material for high-speed modulators due to the strong Pockels effect. Thus, the PZT thin film with a larger EO coefficient was gradually used in modulators when higher-performance devices were needed. In the future, the PZT thin film can also be extended on quartz substrates or heterogeneously integrated with SOI (silicon on insulator) substrates. Therefore, the PZT photonics platform potentially provides a new generation of compact, high-speed, and high bandwidth active devices for telecommunications or other fields (see Note 1 of the supplementary material).

Table I lists and compares several modulator metrics of our device with prior demonstrations, focusing on the EO ferroelectric thin-film modulators. In this discussion, we restricted ourselves to ring resonator structures.

TABLE I.

Comparison of several performance metrics for EO ferroelectric thin-film ring modulators.

Propagation loss (dB/cm)In-device EO coefficient (pm/V)EO tunability (nm/V)High-speed modulation (Gbps)
Ridge LN13  3.0 30 0.007 40 
LN/Si22  NA 30 0.0033 
LN/SiN28  0.2 30 0.0029 NA 
BTO/SiN18  5.6 200 0.0043 20 
BTO/Si20  10 342 0.04 40 
PZT/SiN17  1.0 61 0.0134 40 
This work 1.5 100 0.0358 56 
Ridge PZT 
Propagation loss (dB/cm)In-device EO coefficient (pm/V)EO tunability (nm/V)High-speed modulation (Gbps)
Ridge LN13  3.0 30 0.007 40 
LN/Si22  NA 30 0.0033 
LN/SiN28  0.2 30 0.0029 NA 
BTO/SiN18  5.6 200 0.0043 20 
BTO/Si20  10 342 0.04 40 
PZT/SiN17  1.0 61 0.0134 40 
This work 1.5 100 0.0358 56 
Ridge PZT 

Among them, our PZT offers an advantage of a large EO coefficient over LN, so a higher EO tunability ∆λi/∆V and a lower Vp–p can be realized. Further optimization of the structure of waveguide and electrodes could further reduce the electrical signal loss, thus lowering the device energy consumption and facilitating the integration with CMOS circuits. In comparison with BTO, although the PZT modulator presented in this work has no clear advantage in terms of EO tunability ∆λi/∆V, it features much lower propagation loss (measured by the cut-back method,29 see Note 2 of the supplementary material), indicating a promising potential of being applied in large-scale integrated photonics. The waveguide loss can generally be attributed to two factors: material absorption loss and scattering loss. First, we should prepare crack-free PZT films to minimize material-related losses. Second, we ought to achieve a smooth and contaminant-free waveguide sidewall by optimizing the etching process. More importantly, its modulation speed can achieve up to 56 Gbit/s.

To conclude, we have demonstrated an efficient, high-speed, nanophotonic EO ring modulator on the “PZTOI” platform. The PZT thin film was deposited on a SiO2/Si substrate by using chemical solution deposition. The ridge PZT waveguides enable a strong EO overlap, featuring an ultra-high tuning efficiency of 35.8 pm/V, corresponding to VπL of 1.17 V·cm. Meanwhile, the device exhibited a clear eye diagram of 56 Gbit/s. Together, these results show the suitability of these devices for the on-chip applications. Although we only demonstrated C-band operation in this work, this platform may also be employed into the visible wavelength range.30,31 We believe that the significantly improved EO modulation performance in “EO ferroelectric thin-film on insulator” will lead to a paradigm shift for ultra-high speed optical communication.

The supplementary material contains the simulation bandwidth results of the Mach–Zehnder modulator on the PZTOI platform and the PZT waveguide loss results.

This work was supported by the National Natural Science Foundation of China (Grant No. 62075184), Project funded by China Postdoctoral Science Foundation (Grant No. 2023M740876), and Zhejiang Province Postdoctoral Research Project (Grant No. ZJ2023103). The authors acknowledge Westlake Center for Micro/Nano Fabrication for the facility support and technical assistance.

The authors have no conflicts to disclose.

Guolei Liu: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Writing – original draft (equal); Writing – review & editing (equal). Hongyan Yu: Data curation (lead); Funding acquisition (equal); Formal analysis (equal); Writing – original draft (equal); Writing – review & editing (equal). Dasai Ban: Data curation (equal); Formal analysis (equal). Bin Li: Conceptualization (equal); Writing – review & editing (supporting). Guoqiang Wei: Software (supporting); Writing – review & editing (supporting). Chen Yang: Formal analysis (supporting); Writing – review & editing (supporting). Jungan Wang: Formal analysis (supporting); Investigation (equal). Young-Ik Sohn: Writing – review & editing (equal); Formal analysis (equal). Han Yu: Project administration (equal); Writing – review & editing (equal). Feng Qiu: Conceptualization (supporting); Formal analysis (supporting); Funding acquisition (lead); Project administration (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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